The present invention relates generally to apparatuses for optically reproducing information signals from rotating recording mediums. More particularly, the invention relates to an optically reproducing appratus which, when optically reproducing an information signal from a rotating recording medium on which a pilot or reference signal for tracking control is also recorded together with information signal, simultaneously reproduces also the reference signal and, in response to the reference signal, carries out tracking control of the optically reproducing mechanism.
In general, in an apparatus for optically reproducing an information signal from a rotating recording medium, a light beam is projected onto the disc-shaped rotating recording medium (hereinafter referred to simply as "disc") on which the information signal has been recorded as variations of geometrical shapes on the recording surface of the disc. The light thereby reflected by the recording surface or transmitted through the recording surface is received by light-receiving means, and the light variations responsive to the variations of the geometrical shape of the recording surface are detected and converted into the information signal as the reproduced signal.
An advantageous feature of this optically reproducing apparatus is that it entails no problems whatsoever relating to abrasive wear of the reproducing tracing element and the disc in contrast to reproducing apparatuses of the type wherein a reproducing tracing element is caused to trace by sliding over the disc surface, and the signal recorded as variations in the geometrical shape of the disc surface is thereby picked up as capacitance variations or piezoelectrical variations.
At the time of this optical reproduction, it is necessary to carry out so-called tracking control so that the reproducing light beam will trace accurately and positively the recording track of the disc recording suface.
Among the optically reproducing apparatuses known heretofore, there has been an apparatus of an organization as described below. One example of the disc to be reproduced by this known optically reproducing apparatus has unrecorded parts called "guard bands" interposed between adjacent track turns of a spiral track or circular tracks of concentric form along which the information signal is recorded on the disc as variations of geometrical shape. In an apparatus for reproducing this disc, a single light beam from a light source is divided by the use of a diffraction grating into three light beams, one of which is projected as a reproducing beam onto the center of the recorded track, while the other two light beams are projected as tracking beams onto the disc so as to be positioned on opposite sides of the reproducing beam in the width direction and longitudinal direction of the track. The reproducing apparatus further detects this pair of tracking beams reflected from the disc, thereby detecting any deviation of the reproducing beam center from the center of the recorded track, and carries out tracking control by so controlling the optical system that these two centers coincide.
In this known reproducing apparatus, however, since a pair of beams separate from the reproducing beam is used for the purpose of tracking control, the optical system, including means for dividing a single light beam into three beams, is complicated. Furthermore, the detectors for detecting the beams for tracking control are also required as a pair, whereby the entire organization is complicated. In addition, since a single light beam is divided into three light beams, there arise problems such as a drop in the quantity of light of the reproducing beam. A further problem arises also in the disc to be reproduced by this reproducing apparatus in that, since unrecorded guard bands are provided between adjacent track turns, the efficiency of utilization for recording of the recording surface is poor.